The present invention relates, in a first aspect, to a surgical implant for use with a bone graft between vertebrae, more particularly cervical vertebrae, in order to fuse said vertebrae, to a system comprising said implant and to a method of facilitating the fusion of adjacent vertebrae using said system.
The present invention also relates, in a second aspect, to a surgical fixing screw which may be used for surgical applications generally, but which is particularly suitable for use with the surgical implant according to the first aspect of the invention.
With regard to said first aspect of the present invention, in certain spinal disorders, such as degenerative disease of the intervertebral substance (hereinafter simply referred to as xe2x80x9cdiscxe2x80x9d), it is necessary for the disc to be removed and the neighbouring vertebrae to be fused, aided by a bone graft inserted between the vertebrae. Inevitably, the fusion process is slow, and it is desirable for the bone graft and vertebrae to be immobilised for fusion to take place with the vertebrae and graft correctly positioned.
One solution (exemplified by the so-called Cervical Spine Locking Plate System, available from Synthes) is to insert the bone graft between the vertebrae and then secure the vertebrae to each other by means of a metal plate which lies along one face (generally the anterior face) of the vertebrae. The plate is held in place against the vertebrae by fixing screws which pass through holes provided in the plate and which are screwed into holes drilled into the vertebrae. The bone graft is maintained in position by compressive forces. In use, however, such a plate can be difficult to align properly.
In another system (Fournitures Hospitalieres) a metal plate is integrally formed with a metal ring which protrudes perpendicularly from the midpoint of the plate. The plate can be supplied with a disc of hydroxy-apatite bone substitute fixed in the region enclosed by the ring. The ring containing the hydroxy-apatite disc is inserted between the vertebrae which presents the metal plate in the correct alignment for securing to the vertebrae by means of fixing screws. Alternatively, the plate is supplied without the disc of hydroxy-apatite and the surgeon prepares the bone graft. Difficulties arise if it is necessary to remove the plate, since the entire bone graft will also be removed, requiring the whole procedure to be repeated.
The latter system is not suitable for fusing three adjacent vertebrae.
It is an object of the first aspect of the present invention to provide a surgical implant which facilitates the fusion of adjacent cervical vertebrae, and obviates or mitigates the abovementioned problems.
According to said first aspect of the present invention, there is provided a surgical implant comprising:
(i) a body portion having first and second passages therethrough, said passages being adapted and disposed in said body portion so as to be capable of receiving a securing element (e.g. a screw) for securing said implant to first and second adjacent vertebrae (eg cervical vertebrae) respectively; and
(ii) first and second mutually spaced arms carried by and extending away from said body portion;
wherein said arms are adapted so as to be capable of insertion between said first and second adjacent vertebrae, and wherein at least part of said body portion is adapted to engage with anterior faces of both adjacent vertebrae when said arms are located between said adjacent vertebrae, so that, in use, a bone graft can be held in position between the first and second arms and between mutually facing superior and inferior surfaces of said first and second adjacent vertebrae.
Preferably, the body portion of the implant is constructed such that when the arms of the implant are located between said first and second adjacent vertebrae, another implant can be secured to the anterior face of one of the first and second adjacent vertebrae and a third vertebra, said third vertebra being adjacent to the same one of said first or second adjacent vertebrae.
The arms are preferably straight and preferably also mutually parallel.
Preferably, upper and lower surfaces of each arm are roughened and/or provided with a hydroxy-apatite coating. In use, said surfaces will be in contact with the facing superior and inferior surfaces of the adjacent vertebrae. The roughened surface and/or hydroxy-apatite coating encourages in-growth of bone, thereby providing additional securement of the implant.
The body portion may be provided with more than two passages. Most preferably three or four passages are provided. At least one passage preferably has its axis inclined to the arms. More preferably, at least said first and said second passages have mutually inclined axes.
Preferably, the implant is of a unitary construction of biocompatible material. Examples of suitable materials include titanium, titanium alloy and stainless steel.
Also according to said first aspect of the present invention, there is provided a system for fusing adjacent vertebrae (eg cervical vertebrae), said system comprising at least one surgical implant in accordance with said first aspect, at least one bone graft and securing elements for securing the implant to adjacent vertebrae.
Also according to said first aspect of the present invention, there is provided a method of fusing adjacent vertebrae (eg cervical vertebrae) comprising the steps of:
(i) introducing bone graft material between first and second adjacent vertebrae,
(ii) locating the arms of a surgical implant in accordance with said first aspect around said bone graft material and between said adjacent vertebrae,
(iii) securing at least one securing element into the first vertebra, and
(iv) securing at least one securing element into the second vertebra, wherein each securing element passes through a respective one of said passages in the body portion of the implant.
The method may be extended to the fusion of a third vertebra, said third vertebra being adjacent to one of said first and second adjacent vertebrae, in which case the above steps are repeated with respect to the first and third vertebrae or the second and third vertebrae using another surgical implant.
Preferably, said securing elements are fixing screws of which at least one may be a surgical fixing screw according to said second aspect of the present invention.
With regard to said second aspect of the present invention, it is an objection to provide a design of surgical fixing screw which enables a secure fixing of a part such as an implant to take place with a reduced risk of disengagement from such part.
According to said second aspect of the present invention, there is provided a surgical fixing screw including a head and a screw-threaded shank, wherein the head is formed so that it can be transformed between a laterally expanded condition and a laterally contracted condition.
The term xe2x80x9claterallyxe2x80x9d means laterally relative to the longitudinal axis of the shank. Preferably, the head is formed so that it can be transformed between a radially expanded condition and a radially contracted condition.
Preferably, the surgical fixing screw further includes retaining means engageable within the head for preventing the head from being transformed into its laterally contracted condition.
When in its laterally contracted condition, the surgical fixing screw can be inserted within an aperture in the part to be secured. When in its outwardly extended condition, the head of the surgical fixing screw enables fixing of the screw within the aperture in the part in a manner such as to reduce the risk of disengagement of the surgical fixing screw from the part.
In a preferred embodiment, the head is shaped so that, in its laterally expanded condition, it projects laterally into an undercut recess in the part whereby the head is prevented from disengagement from the part by interlocking with the latter.
Preferably, the head of the surgical fixing screw has a recess therein in which the retaining means is engageable so that the retaining means engages a wall of the recess to prevent the head from being transformed into its laterally compressed condition.
Also according to said second aspect of the invention, there is provided a surgical fixing screw comprising a head and an externally screw-threaded shank which is divided longitudinally into lateral wings which are capable of being deformed outwardly of the longitudinal axis of the shank. This obviates or mitigates the risk of the screw becoming detached from, or loosened in, a hole in bone or hard tissue in which the screw shank is engaged in use.
Conveniently, the shank is adapted to receive an expansion means which, during insertion into the fixing screw, causes the outward deformation of the wings. Desirably, the body of the shank tapers inwardly away from the head.
Preferably, the expansion means is held within the shank by means of a complementary thread arrangement.
Most preferably, the expansion means for causing outward deformation of the wings and the retaining means for the head are provided as different parts of a single retaining and expansion element engageable with the shank and the head of the screw.
Preferably, the surgical fixing screw is constructed and adapted for use in the securing of a surgical implant in position.
The head of the screw may have a first formation for engagement, in use, by a screwdriver and a second formation, spatially distinct from the first formation, for engagement, in use, by an alternative screwdriver. Thus, in the event that one of the first and second formations becomes damaged during insertion or removal of the screw using the appropriate screwdriver, the screw can be removed subsequently using the other screwdriver on the other of the first and second formations.
Desirably, said first and second formations are spaced apart longitudinally in the axis of rotation of the surgical fixing screw.